1. Field of the Invention
The present invention relates to electrohydrodynamic printing and manufacturing techniques and their application in liquid drop/particle and fiber production, colloidal deployment and assembly, and composite materials processing.
2. Discussion of the Background
Processing and conversion of micro- and nano-structural building blocks such as particles and fibers into composite materials and functional devices is essential for practical applications of micro and nanotechnology. Bottoms-up and top-down paradigms are complementary in their accessible length scales. However, contemporary techniques for fabricating microscale structures usually emphasize one aspect only, for example, self assembly covers the nanometer-scale from the bottom-up; pick-and-place covers the micrometer-scale from the top-down. Electrohydrodynamic (EHD) printing is a new paradigm for micro- and nano-manufacturing that can be used in two distinct modes to deploy either jets or drops onto surfaces. This EHD approach takes advantage of the large neck-down ratio of the cone-jet transition, which enables the production of nano- to micron-scale jets and/or drops from millimeter-scale nozzles and thus eliminates the nozzle clogging problem. Since the solutions used to create the jets and/or the drops can be self-assembling systems, these deployment techniques integrate the merits of both pick-and-place and self assembly into a single operation. The idea is to deploy liquid drops or jets containing self-assemblying particles to patterned locations through colloidal jets and/or drops and utilize these as building blocks for complex structures.
Using EHD printing, micro and nanostructures can be built through either one and/or combination of the following procedures:                i. Fiber by fiber by deploying liquid jets (e.g., structural nanocomposites);        ii. Particle by particle by deploying one particle per drop (e.g. photonic waveguide);        iii. Self assembly within the deployed fibers or drops (e.g. self-healing ceramic thermal insulation foam).Compared to contemporary manufacturing techniques, the EHD printing technique is unique in that it eliminates tedious and costly cleanroom processes using the cone-jet transition and facilitates self assembly by carrying colloidal particles within EHD suspensions.        
In fiber production, electrospinning is also an application of electrohydrodynamic cone-jet transition which relies on EHD whipping instabilities to stretch the electrified jets to produce thin polymeric fibers. These whipping instabilities lead to poor control of fiber orientation and usually result in polymeric mats with randomly oriented fibers. Although conventionally electrospinning is used to produce a very high surface area mat of randomly distributed fibers, which is used in applications such as filtering, protective clothing and tissue scaffolding; recently, there have been numerous techniques proposed to orient electrospun fibers by modifying the collector, which also works as a counter electrode. Two categories of collector modification are reported: (i) changing the shape of counter electrodes and direct the polymeric fiber along the direction of electric field; reported shapes include ring, edge, frame and parallel-strips; (ii) rotating the collector and deposit the polymeric fiber along the direction of rotation; reported configurations include rotating drum and plate. Although parallel or crossed line patterns can be achieved, these methods cannot be applied to more complex patterns. For complex pattern formation, the impingement of the filament to the target point should be controlled with high accuracy and precision.
A few electrospinning studies suggest using electrode separations smaller than conventional separations used in electrospinning. Natarajan et al. used 1-3 cm electrode separations together with point like bottom electrode to achieve aligned fibers. Craighead et al. produced aligned nano fibers on conducting/non-conducting striped substrates using 1 cm electrode separation. Although these authors used small electrode separations, they did not pay attention to the stability of the EHD filament. The main concern of these authors regarding electrode separation was solvent evaporation rather than stability. They avoided separations shorter than 1 cm. because membrane formation was observed for shorter separations rather than fiber formation. That they obtain a membrane and not linear patterns on a moving substrate is an indication of unstable nature of the EHD filament in their system. Because there is no set electrode separation for obtaining a straight and intact filament; oscillations of the filament may set in at separations as low as a few millimeters. In fact, Craighead and coworkers also reported that deposited fibers were not straight unless the rotary table speed is larger than a critical value, which suggests that at their operating conditions the filament was oscillatory.
In drop production, pulsed EHD jetting may be the only drop generation technique that can produce drops on-demand with dimensions a decade or so smaller than the nozzle. Although ‘on-demand’ drops are readily produced by an external voltage pulse, the large neck-down ratio derives from the EHD cone-jet transition which is fundamental to electrospray ionization. EHD cone-jets pulsate in response to intrinsic processes or external stimuli. Two intrinsic pulsating modes can arise due to the imbalance between the supply and loss of liquid in the entire cone volume (low frequencies) or in the cone's apex (high frequency). Externally pulsed electrosprays achieve higher sensitivity and better signal-to-noise ratio compared to the steady counterpart. Externally pulsed cone-jets were also exploited by to generate pico- to femtoliter droplets.
Contemporary techniques for particle deployment can be roughly classified as robotic, lithography-directed, and field-directed. Robotic manipulation is accomplished using MEMS effectors for pick-and-place or scanning probes like AFM tips; this category offers direct manipulation at nanoscale but has contact contamination and low throughput. Lithography-directed manipulation uses microfabricated patterns to guide particle deployment; this category offers batch manipulation but spatial resolution is limited and the technique is somewhat inflexibile due to the use of fixed lithographic patterns. Field-directed manipulation relies on field gradients to trap and move objects (e.g., optical tweezers); this category offers non-intrusive manipulation but the type of particle and operating environments are restricted. The EHD line printing and/or drop-and-place techniques aim at deploying particles via colloidal jets and/or droplets. EHD drop-and-place and fiber deployment can circumvent the aforementioned drawbacks and achieve flexible, non-contact manipulation of a variety of materials at relatively high precision (sub-micron) and high speed (kilo-Hertz).